Simulcast communication systems are known to comprise transmitters, a plurality of communication units, such as in-car mobile and/or hand-held portable radios, and a central simulcast controller. In such a system, a minimum number of transmitters, typically distributed throughout a large geographic region, are synchronized under the control of the central simulcast controller to provide the most uniform radio frequency (RF) coverage area possible. By fixing the transmission delays between the transmitters, communication units may travel over very large distances and still retain the ability to communicate, at acceptable quality levels, within the simulcast system.
It is known, however, that various locations within the coverage area of a simulcast system can provide widely fluctuating reception quality due to superposition of RF waves of varying amplitudes and delays. When a communication unit operates in close proximity to a transmitter, reception by the communication unit is clear because interference from the other transmitters is overpowered by the nearby transmitter. In those areas where signals from multiple transmitters are substantially equal in power, the quality of received communications can vary due to differential delays between the multiple signals. That is, when the differential delays are substantially zero, the multiple signals constructively interfere with each other thereby providing good audio reception for communication units within the area. Significant differential delays within such areas can cause the multiple signals to destructively interfere with each other, thus reducing the quality of received signals to the point of complete unintelligibility. Such delays are caused for a variety of reasons, such as topographical variations within the coverage area. Thus, communication units may travel only small distances and experience dramatic reductions in received audio quality.
A solution to this problem is to increase the transmit power of some of the transmitters to improve reception within the original coverage area. This solution can cause further problems, especially to the power amplifiers (PA) used by the transmitters. Transmitters, and their associated PA's, operating at higher power levels for increased periods of time can cause degraded performance or lead to premature failure. Also, in simulcast systems that transmit an RF modulation scheme of a non-constant envelope, higher operating powers could cause PA's to operate outside their linear range, causing unwanted distortion. Thus, a need exists for a method which allows signal quality to be enhanced in those areas of simulcast systems experiencing low signal quality without the use of higher power levels.